Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Goodwill [Line Items]
Beginning balance	$ 7,341	$ 6,674
Goodwill of acquired businesses	698	621
Goodwill adjustment during measurement period	9	(12)
Goodwill disposed of relating to sales of businesses	(13)	(47)
Foreign currency translation adjustments during the year	(65)	105
Ending balance	7,970	7,341
Retail
Goodwill [Line Items]
Beginning balance	4,870	4,436
Goodwill of acquired businesses	671	335
Goodwill adjustment during measurement period	(37)	13
Goodwill disposed of relating to sales of businesses	(13)	0
Foreign currency translation adjustments during the year	(55)	86
Ending balance	5,436	4,870
Programs
Goodwill [Line Items]
Beginning balance	1,853	1,646
Goodwill of acquired businesses	0	263
Goodwill adjustment during measurement period	40	(23)
Goodwill disposed of relating to sales of businesses	0	(47)
Foreign currency translation adjustments during the year	(9)	14
Ending balance	1,884	1,853
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	618	592
Goodwill of acquired businesses	27	23
Goodwill adjustment during measurement period	6	(2)
Goodwill disposed of relating to sales of businesses	0	0
Foreign currency translation adjustments during the year	(1)	5
Ending balance	$ 650	$ 618